The invention relates to a method of manufacturing tube glass, in which following the drawing of a tube from molten glass the inside surface of the tube is treated with a chemically reactive gas or gas mixture.
Methods of this type are known per se.
Reference can be made to, for example, U.S. Pat. No. 4,717,607 in which a description is given of a method of extracting alkali and alkaline earth ions from the surface of tube glass during the drawing of tube glass from a glass melt. In said known method the reactive gas consists of a mixture of a gaseous organo-fluoride compound and an oxidizing gas. The reaction is brought about by the high temperature of the glass in the so-called bag of soft glass. The fluoride-containing gas formed reacts with the alkali and alkaline earth ions in the glass surface. The alkali and alkaline earth compounds formed are exhausted via the end of the tube formed from the glass. The tube glass obtained by this method can be suitably used for the manufacture of fluorescent lamps having a mercury-containing atmosphere.
A similar method is known from the published Netherlands Patent Application NL-A-79 06 006, which corresponds to British Application 2,027,689. In said method, for example, a volatile chloride of a metal and a gaseous oxidation means are dispensed into the bag of soft glass during drawing. Under the influence of the high temperature in the bag (800.degree.-950.degree. C.) a film of the oxide of the metal in question would be deposited on the inside wall of the bag. It is known, however, that a reaction between, for example, oxygen and the chlorides of, for example, silicon, titanium, tin only takes place in this temperature range when the reaction mixture contains a quantity of hydrogen or water vapour. In said Patent Application, layers of tin oxide, titanium oxide and indium oxide are mentioned as examples of layers which can be applied.
The known methods have the disadvantage that the surface layers formed in the bag of soft glass are deformed during the drawing of the tube to a smaller diameter. When the layers formed have a higher melting temperature or a higher softening temperature than the underlying glass, cracks may be formed in said layers as a result of the unavoidable deformation of said layers during the shaping of the bag into a tube. Due to this, the intended protection of the underlying glass or the intended optical function of the layer applied are not optimally obtained.